1. Field
This invention relates in general to the field of pressurized light water nuclear reactors and, in particular, to radial neutron reflectors which surround the nuclear core for improved neutron economy to lower fuel costs and reduce the radiation dose on the pressure vessel.
2. Description of Related Art
The fission reactions in a nuclear reactor generate heat and release neutrons which produce additional fission reactions in the nuclear fuel. The fissile material is massed in the reactor such that the neutron flux density is sufficient to maintain a sustained fission process. In a commercial reactor, pellets of the fissile material are encased in Zircoloy rods mounted in modular, elongated fuel assemblies which are generally square in cross section. A large number of these square, elongated fuel assemblies are massed to form a generally cylindrical reactor core which is housed inside the cylindrical stainless steel core barrel between horizontal upper and lower stainless steel core plates. This entire assembly, in turn, is mounted inside a pressure vessel with generally hemispherical upper and lower heads. Reactor coolant, introduced into the pressure vessel through inlet nozzles, flows downward in an annular space between the core barrel and the pressure vessel, reverses direction in the lower plenum of the vessel, flows upward through openings in the lower core plate, and through the fuel assemblies where it is heated as a result of the fission reactions before being directed radially out of the pressure vessel through outlet nozzles. The heat extracted by the reactor coolant from the core is utilized to generate electricity thereby lowering the temperature of the reactor coolant which is recirculated through the reactor in a closed loop.
Since the fuel assemblies are square in cross section, an irregular space exists between the periphery of the core and the round inner surface of the core barrel. The usual practice is to place longitudinally extending flat, baffle plates along the outer surfaces of the fuel assemblies to confine the upward coolant flow to the fuel assemblies. The baffle plates are held in place by horizontal, irregularly shaped former plates that are bolted to and between the longitudinal baffle plates and the core barrel. Holes in the former plates permit limited coolant flow in the generally annular space between the longitudinal baffle plates and the core barrel to provide cooling for these components and to equalize the pressure on both sides of the longitudinal baffle plates.
Although the original purpose of the vertical baffle plates was to channel reactor coolant flow through the fuel assemblies, it has been recognized that to some extent they also reflect neutrons back towards the peripheral fuel assemblies. However, since these plates are relatively thin, most of the neutrons escaping radially from the core migrate into the large volume of water between the baffle plates and the core barrel which absorbs or thermalizes the neutrons with very little reflection.
Radial reflectors are designed to reflect neutron radiation back into the interior active core region of the reactor vessels during power operation to improve the efficiency of the reactors and to protect the reactor vessels from the embrittling affects of years of irradiation during power operation.
With an aging fleet of reactors around the world, there is a current need to extend the life of reactor vessels by more effectively shielding the vessel walls from irradiation to satisfy the requirements for plant licensing extensions. Furthermore, new reactors are being designed with larger and larger cores that will necessitate a more effective shield of the reactor vessel wall to achieve the desired operating life.
Accordingly, it is an object of this invention to provide a radial reflector design that provides improved protection for the reactor vessel walls against radiation that will extend the vessel's life.
It is a further object of this invention to provide a new radial reflector design that can economically be constructed, and preferably back-fitted into existing reactors.
Further, it is an additional object of this invention to provide such a radial reflector design that can accommodate ever larger reactor cores with minimum increases in reactor vessel size.